Abstract
Highly pathogenic avian influenza (HPAI) virus clade 2.3.4.4b genotype B3.13 infected the mammary gland of dairy cattle; the new tissue tropism and host heightened concern about its ability to cross species barriers with zoonotic potential. Pigs play a key role in influenza A virus (IAV) adaptation, serving as a "mixing vessel" for the emergence of reassortants. The susceptibility of porcine mammary gland to HPAI infection remains unexplored. In this study, we profiled IAV receptors in porcine mammary gland as well as respiratory tract tissues. Additionally, we evaluated the binding capacity of IAVs to these tissues. Furthermore, we isolated primary cells from porcine mammary gland and respiratory tract, and immortalized them. We examined the growth potential of IAV isolates from bovine, avian, swine, and human on these cells. We showed that porcine mammary gland displays both SA-α2,3 and SA-α2,6, and that IAVs bind to mammary gland tissues with variable affinities. While bovine H5N1 virus replicates efficiently in mammary gland and respiratory tract cells, replication of other IAVs in mammary epithelial cells is moderate but is efficient in respiratory cells. These findings suggest that porcine mammary gland could support the infection by HPAI 2.3.4.4b genotype B3.13. Our findings expand the current understanding of IAV tissue tropism in swine by revealing that the porcine mammary gland may serve as an alternative and previously overlooked replication site for both avian and mammalian-origin viruses. Given the increasing frequency of interspecies spillover events and expanding host range of HPAI H5N1, our finding that mammary epithelial cells have an intrinsic capacity to support replication of diverse IAV strains provides new insight into the porcine mammary gland as a potential replication and reassortment site for IAV.
